Electrochromic devices such as electrochromic mirrors and electrochromic windows are by now well known. Electrochromic devices generally contain at least two electroactive compounds, at least one of which exhibits absorbance in the visible spectrum in its oxidized or reduced state. By the term "electroactive" is meant a compound which is capable of being oxidized or reduced by application of an electric potential. By the term "electrochromic" is meant any electroactive compound which exhibits a change in color or absorbancy when oxidized or reduced.
Electrochromic devices, and electrochromic media suitable for use therein, are the subject of numerous U.S. patents, including U.S. Pat. No. 4,902,108, entitled "Single-Compartment, Self-Erasing, Solution-Phase Electrochromic Devices, Solutions for Use Therein, and Uses Thereof", issued Feb. 20, 1990 to H. J. Byker; Canadian Pat. No. 1,300,945, entitled "Automatic Rearview Mirror System for Automotive Vehicles", issued May 19, 1992 to J. H. Bechtel et al.; U.S. Pat. No. 5,128,799, entitled "Variable Reflectance Motor Vehicle Mirror", issued Jul. 7, 1992 to H. J. Byker; U.S. Pat. No. 5,202,787, entitled "Electro-Optic Device:, issued Apr. 13, 1993 to H. J. Byker et al.; U.S. Pat. No. 5,204,778, entitled "Control System For Automatic Rearview Mirrors", issued Apr. 20, 1993 to J. H. Bechtel; U.S. Pat. No. 5,278,693, entitled "Tinted Solution-Phase Electrochromic Mirrors", issued Jan. 11, 1994 to D. A. Theiste et al.; U.S. Pat. No. 5,280,380, entitled "UV-Stabilized Compositions and Methods", issued Jan. 18, 1994 to H. J. Byker; U.S. Pat. No. 5,282,077, entitled "Variable Reflectance Mirror", issued Jan. 25, 1994 to H. J. Byker; U.S. Pat. No. 5,294,376, entitled "Bipyridinium Salt Solutions", issued Mar. 15, 1994 to H. J. Byker; U.S. Pat. No. 5,336,448, entitled "Electrochromic Devices with Bipyridinium Salt Solutions", issued Aug. 9, 1994 to H. J. Byker; U.S. Pat. No. 5,434,407, entitled "Automatic Rearview Mirror Incorporating Light Pipe", issued Jan. 18, 1995 to F. T. Bauer et al.; U.S. Pat. No. 5,448,397, entitled "Outside Automatic Rearview Mirror for Automotive Vehicles", issued Sep. 5, 1995 to W. L. Tonar; and U.S. Pat. No. 5,451,822, entitled "Electronic Control System", issued Sep. 19, 1995 to J. H. Bechtel et al., each of which patents is assigned to the assignee of the present invention and the disclosures of each of which are hereby incorporated herein by reference, are typical of modern day automatic rearview mirrors for motor vehicles. These patent references describe electrochromic devices, their manufacture, and electrochromic compounds useful therein, in great detail.
While numerous electrochromic devices are possible, the greatest interest and commercial importance are associated with electrochromic windows, light filters and mirrors. A brief discussion of these devices will help to facilitate an understanding of the present invention.
Electrochromic devices are, in general, prepared from two parallel substrates coated on their inner surfaces with conductive coatings, at least one of which is transparent such as tin oxide, or the like. Additional transparent conductive materials include fluorine doped tin oxide (FTO), tin doped indium oxide (ITO), ITO/metal/ITO (IMI) as disclosed in "Transparent Conductive Multilayer-Systems for FPD Applications", by J. Stollenwerk, B. Ocker, K. H. Kretschmer of LEYBOLD AG, Alzenau, Germany, and the materials described in above-referenced U.S. Pat. No. 5,202,787, such as TEC 20 or TEC 15, available from Libbey Owens-Ford Co. (LOF) of Toledo, OH. Co-filed U.S. Pat. Appln. entitled "AN IMPROVED ELECTRO-OPTIC DEVICE INCLUDING A LOW SHEET RESISTANCE, HIGH TRANSMISSION TRANSPARENT ELECTRODE" describes a low sheet resistance, high transmission, scratch resistant transparent electrode that forms strong bonds with adhesives, is not oxygen sensitive, and can be bent to form convex or aspheric electro-optic mirror elements or tempered in air without adverse side effects. The disclosure of this commonly assigned application is hereby incorporated herein by reference.
The two substrates of the device are separated by a gap or "cavity", into which is introduced the electrochromic medium. This medium contains at least one anodic or cathodic electrochromic compound which changes color upon electrochemical oxidation or reduction, and at least one additional electroactive species which may be reduced or oxidized to maintain charge neutrality. Upon application of a suitable voltage between the electrodes, the electroactive compounds are oxidized or reduced depending upon their redox type, changing the color of the electrochromic medium. In most applications, the electroactive compounds are electrochromic compounds which change from a colorless or near colorless state to a colored state. Upon removal of the potential difference between the electrodes, the electrochemically activated redox states of electroactive compounds react, becoming colorless again, and "clearing" the window.
Many improvements to electrochromic devices have been made. For example, use of a gel as a component of the electrochromic medium, as disclosed in U.S. Pat. Nos. 5,679,283 and 5,888,431, both entitled "Electrochromic Layer and Devices Comprising Same", and U.S. application Ser. No. 08/616,967, entitled "Improved Electrochromic Layer And Devices Comprising Same", have allowed the preparation of larger devices which are also less subject to hydrostatic pressure.
In electrochromic mirrors, devices are constructed with a reflecting surface located on the outer surface of the substrate which is most remote from the incident light (i.e. the back surface of the mirror), or on the inner surface of the substrate most remote from the incident light. Thus, light striking the mirror passes through the front substrate and its inner transparent conductive layer, through the electrochromic medium contained in the cavity defined by the two substrates, and is reflected back from a reflective surface as described previously. Application of voltage across the inner conductive coatings results in a change of the light reflectance of the mirror.
In electrochromic devices, the selection of the components of the electrochromic medium is critical. The medium must be capable of reversible color changes over a life cycle of many years, including cases where the device is subject to high temperatures as well as exposure to ultraviolet light. Thus, the industry constantly seeks new electrochromic media and new electroactive compounds which will resist aging, particularly in exterior locations. The effects of ultraviolet light, in particular, are felt more strongly when the electroactive compounds contained in electrochromic media are energized to their respective oxidized and reduced states.
In many applications, for example electrochromic mirrors, it is desirable that the mirror, both in its inactive as well as its active state, be a relatively neutral color, for example gray. In addition, it is desirable that the color can be maintained over a range of voltage, for example, that the absorbance of the electrochromic medium may be changed without undesirably changing the hue, in particular between "full dark" and "clear" conditions.
Prior art electrochromic media generally employed two electrochromic compounds, one anodic and one cathodic, and were unable to acceptably produce gray shades, and numerous other shades of color as well. In U.S. application Ser. No. 08/837,596 filed Apr. 2, 1997, herein incorporated by reference, non-staging devices capable of achieving a preselected color are disclosed. These devices contain at least three active materials, at least two of which are electrochromic compounds, and exhibit little or no staging while being available in neutral colors such as gray, or in other preselected colors not normally available.
To maintain electrical neutrality in electrochromic devices, for each oxidation involving a single electron at the anode, a corresponding reduction must occur at the cathode. Moreover, as the number of electrons transferred at each of the two electrodes must be the same, a two electron event occurring at one electrode must be balanced by either two single electron events or a single two electron event at the opposing electrode.
While in principle it is possible for an electrochromic device to contain only one electrochromic compound together with an electroactive compound which is colorless in both the unactivated and activated states, in the majority of devices, both the anodic electroactive compound and the cathodic electroactive compound are electrochromic compounds. In this way, colored species are generated at each electrode. Thus, the coloration is intensified, at the same current level, by employing two electrochromic compounds as opposed to one electrochromic compound and one colorless electroactive compound.
A significant improvement in the stability of electrochromic devices is disclosed in U.S. Pat. No. 4,902,108 which employs electrochromic compounds displaying two chemically reversible waves in a cyclic voltammogram. Such compounds have minimally two electrochemically activated states. The observation of a second chemically reversible wave is an indication that the second electrochemically activated state is reasonably stable.
When employing electrochromic compounds which display two chemically reversible waves in their cyclic votammograms, the device potential is generally set to generate species of the first electrochemically activated state only. However, in these devices, higher redox state species are created by disproportionation of two species in the first electrochemically activated state, for example, 2A.sup.+ A.sup.0 +A.sup.2+. Because of the higher potential of the 2+ species, the equilibrium lies to the left. However, because the 2+ species is more reactive, and more subject to irreversible chemical change, the continual removal of this species, even though ordinarily present in extremely small quantities, can result in the long term degradation of device performance. Thus, it is still desirable to improve the stability of electrochromic devices, both those containing electrochromic compounds having but a single electrochemically activated state as well as those displaying a two or more sets of waves in a cyclic voltammogram, whether chemically reversible or irreversible.
It has been desirable to limit the amounts of the electrochemically activated states of the electroactive compounds to sufficient amounts such that the degree of absorbance required for the device was obtained. As discussed previously, for every electron transferred at the cathode in the reduction of the cathodic material will be matched at the anode by the transfer of an electron involved in the oxidation of the anodic material. In other words, the number of moles of electrons transferred at the cathode will equal the number of moles of electrons transferred at the anode. This condition applies to the rates of electron transfer (the current passed) as well as the total number of electrons transferred. The current of an electrochemical reaction is related to the diffusion of the material being oxidized or reduced as well as its concentration or abundance. For example, in "A CALCULATION OF STEADY STATE ELECTROCOLORATION PARAMETERS ON ELECTROCHROMIC SYSTEMS" an equation relating current density to the diffusion coefficients and concentrations is given. In electrochromic devices the diffusion rates for the various redox forms of the materials are, generally different. Therefore the actual amounts of anodic and cathodic materials required to achieve this balanced current condition where no excess anodic materials are present at the anode nor excess cathodic materials present at the cathode will differ from a 1:1 ratio. In general a mole ratio of less mobile electroactive material to the more mobile material will be greater than 1:1 to achieve current balance. The concentrations of electroactive materials required to achieve this current balance may be termed the "balanced concentrations".